1. Field of the Invention
The present invention relates to an information recording medium having a substrate manufactured by injection molding, and a method of manufacturing such an information recording medium.
2. Description of the Related Art
Optical information recording mediums (optical disks) capable of recording information once with a laser beam include a write-once CD (so-called CD-R) and a DVD-R. These optical information recording mediums are advantageous in that they allow a smaller quantity of CDs to be supplied quickly to the market at a reasonable price than conventional CDs (compact disks), and there are growing demands for such optical information recording mediums in view of the recent widespread use of personal computers.
Typically, a CD-R-type information recording medium comprises a disk-shaped transparent substrate having a thickness of about 1.2 mm, a recording layer of an organic dye deposited on the substrate, a light reflecting layer of metal such as gold disposed on the recording layer, and a protective layer of resin disposed on the light reflecting layer. For details, see Japanese laid-open patent publication No. 6-150371, for example.
A DVD-R-type information recording medium comprises two disk-shaped transparent substrates each having a thickness of about 0.6 mm which are bonded to each other with their information recording surfaces facing each other. The DVD-R-type information recording medium can record a greater amount of information.
The substrates of these optical disks are generally made of polycarbonate or acrylic resin by injection molding or injection compression molding for better productivity.
Specifically, a stamper is disposed in a cavity defined between a fixed mold and a movable mold of a closed mold assembly of an injection molding machine, and a molten resin is injected into the cavity to produce a substrate which has tracking grooves and recesses and protrusions representing information such as address signals transferred to its surface from the stamper.
Usually, one or two substrates are manufactured in one injection molding cycle. Substrates which have been molded are removed from the mold and cooled. Specifically, the substrates are cooled by either being placed flatwise on respective rotary tables which having chucks for holding the substrates at central holes thereof, or being arranged vertically in a magazine.
For manufacturing CD-R-type information recording mediums, two substrates are simultaneously molded, and the molded substrates are cooled on independent lines. For manufacturing DVD-R-type information recording mediums, one of the two substrates to be bonded together is supplied from a stock of substrates.
According to the above process of manufacturing CD-R-type information recording mediums, since two substrates are simultaneously molded, and the molded substrates are cooled on independent lines, there is developed a temperature difference between the substrates that have been cooled on the independent lines. Therefore, the substrates tend to suffer warpage or swaying, and resulting in the development of a performance difference when films are grown thereon.
According to the above process of manufacturing DVD-R-type information recording mediums, there is developed a temperature difference between two bonded substrates, i.e., a substrate from the stock and an injection-molded substrate with a dye recording layer formed thereon, and hence the substrates tend to suffer warpage or swaying.